A conventional projector is known, which modulates light emitted from a lighting device by a spatial light modulator and projects onto a screen, an enlarged form of modulated image light through a projection optical system, such as projection lens. Conventionally, a metal hydride lamp, halogen lamp, etc. has been used as the lighting device of the projector. In recent years, however, use of a laser diode (LD) has been proposed in order to reduce the size of the lighting device and the projector.
However, because laser light is coherent light, video light projected in an enlarged form develops speckle patterns having randomly distributed luminous dots and dark dots. The speckle patterns are created when beams of light emitted from different points of the projection optical system interfere with each other because of the irregular phase relation between the beams of light. Such speckle patterns deteriorate the quality of a projected image or video. As a solution to such a problem, an image display device is known, which reduces speckle noise by controlling the polarized state of each beam of laser light emitted from each of primary color light source (see, e.g., Japanese Patent Laid-Open Publication Nos. 2010-160307 and 2007-121842). Another image display device is also widely known, which controls the polarization state of light and has a 3D display function.    Patent document 1: Japanese Patent Laid-Open Publication No. 2010-160307    Patent document 2: Japanese Patent Laid-Open Publication No. 2007-121842